Increasing contrast only in Y' (luma) can make color appear less saturated because perceived saturation depends roughly on the chroma-to-luma ratio. If luma contrast increases while chroma stays the same, that ratio drops in affected areas, so the image can look less saturated even though the color channels were not directly reduced.

As a first-order approximation, keeping perceived saturation similar means keeping chroma contrast and luma contrast in balance rather than changing only one.

The daylight example is not quite the same phenomenon. Colors seeming less saturated at midday versus dawn or dusk is better explained by human color appearance, specifically the Hunt effect, rather than just simple luma-contrast math. In other words, image-processing channel relationships and human visual perception are related but not identical explanations.